1. Field of the Invention
This invention relates to molded case circuit breakers and more particularly to a crossbar assembly having welded contact arm carriers and molded insulating sleeves pinned to the crossbar to prevent axial movement due to magnetic repulsion forces generated during overcurrent conditions and a form wound shunt.
2. Description of the Prior Art
Molded case circuit breakers are generally old and well known in the art. Examples of such circuit breakers are disclosed in U.S. Patent Nos. 4,489,295; 4,638,277; 4,656,444 and 4,679,018. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and relatively high level short circuit. An overload condition is about 200-300% of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000% or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers include at least one pair of separable contacts which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation the contacts may be opened by an operating mechanism or by a magnetic repulsion member. The magnetic repulsion member causes the contacts to separate under relatively high level short circuit conditions. More particularly, the magnetic repulsion member is connected between a pivotally mounted contact arm and a stationary conductor. The magnetic repulsion member is a generally V-shaped member defining two legs. During high level short circuit conditions, magnetic repulsion forces are generated between the legs of the magnetic repulsion member as a result of the current flowing therethrough which, in turn, causes the pivotally mounted contact arm to open.
In a multipole circuit breaker, such as a three-pole circuit breaker, three separate contact assemblies having magnetic repulsion members are provided; one for each pole. The contact arm assemblies are operated independently by the magnetic repulsion members. For example, for a high level short circuit on the A phase, only the A phase contacts would be blown open by its respective magnetic repulsion member. The magnetic repulsion members for the B and C phases would be unaffected by the operation of the A phase contact assembly. The circuit breaker operating mechanism is used to trip the other two poles in such a situation. This is done to prevent a condition known as single phasing, which can occur for circuit breakers connected to rotational loads, such as motors. In such a situation, unless all phases are tripped, the motor may act as a generator and feed the fault.
In the other automatic mode of operation, the contact assemblies for all three poles are tripped together by a current sensing circuit and a mechanical operating mechanism. More particularly, current transformers are provided within the circuit breaker housing to sense overcurrent conditions. When an overcurrent condition is sensed, the current transformers provide a signal to electronic circuitry which actuates the operating mechanism to cause the contacts to be separated.
A crossbar assembly is mechanically coupled to the operating mechanism for the circuit breaker. The crossbar assembly contains a pair of contact arm carriers which connect to a toggle assembly which forms a portion of the operating mechanism. The movable contact assemblies, which carry the movable contacts, are mechanically coupled to the crossbar by way of a cam roll pin assembly. During overcurrent conditions less than the withstand rating of the circuit breaker, the crossbar assembly and the cam roll pin assembly open all three poles in a three pole breaker simultaneously. During an overcurrent condition greater than the withstand rating of the circuit breaker one or more poles are tripped by the magnetic repulsion members. The crossbar assembly subsequently trips the remaining poles.
Since the crossbar assembly is in contact with current carrying components, the crossbar is insulated to minimize the magnetic repulsion forces generated between adjacent poles. Conventional crossbar assemblies are formed from an elongated steel bar. Insulating paper is compressed and baked onto the crossbar. The contact arm carriers are then slid onto the crossbar and stapled in place. If the contact arm carriers are forced on or the stapling procedure is made too tightly, the insulation can crack resulting in a dielectric failure. On the other hand, if the contact arm carriers are not stapled tightly enough the contact arm carriers can loosen due to magnetic repulsion forces generated during an overcurrent condition and eventually fail to support the contact arms.